Protein backbone NH exchange kinetics in folded globular proteins demonstrate that although interior NH protons are shielded from solvent, there are conformational motions by which most buried backbone NH protons are rendered accessible to solvent OH negative and OH positive ions and H2O. Understanding the nature of these motions is the object of hydrogen exchange studies. In the year 1981-1982, we plan to measure the hydrogen-deuterium exchange rates of the "more rapidly exchanging" protons in BPTI by NMR. These are the protons that exchange more rapidly than the 7 slowest exchanging protons for which there are extensive data. The more rapidly exchanging protons are of greater interest because for them we will be able to measure over the entire pH range under conditions where unfolding does not contribute to exchange. Then we will be able to determine if, for example, the pH dependence for exchange from the folded state is the same as for model peptides. If so, then it is reasonable to assume that the exchange expression for the folded state the chemical exchange rate is the same as for model compounds. Therefore we can calculate for 5-7 assigned protons, the equilibrium probability for their exposure to water and OH negative ion. If, as we propose, exchange occurs in the protein matrix, the pH dependence of protons in the native protein, as compared to model compounds, has important implications for the types of protein motions limiting exchange, as they must allow access of buried NH's to OH negative ion as well as to H2O. We also plan to measure the exchange rates of protons trapped at the protein-protein interface of the trypsin-BPTI complex. Again we use partial labeling, as described in the original grant, to label the interface protons with 1H. The protons buried inside both trypsin and BPTI are deuterated and therefore not observed in the proton NMR spectrum.